This specification describes, with reference to the accompanying drawings, a novel and improved high impedance circuit. While this circuit may be used in any application requiring a high impedance, low noise circuit, the description below refers specifically to its use as an input bias circuit for a preamplifier circuit for a microphone component of a hearing aid.
Generally speaking, a hearing aid utilizes three basic components, a microphone, an amplifier and a receiver or loudspeaker component. Briefly, the microphone generally includes a transducer for converting incoming sound pressure levels to corresponding electrical signals. The amplifier amplifies these electrical signals to a desired level, and the receiver or loudspeaker component translates these amplified electrical signals to an acoustical sound output, or outgoing sound pressure level.
Generally, the microphone component includes a transducer such as an electret microphone, and a buffer amplifier or “preamplifier.” Often, a CMOS amplifier component is used as the preamplifier. The input of this CMOS preamplifier must be biased to ground level. However, since the signal source in the electret microphone is typically a relatively small capacitor, the impedance of the preamplifier input should be relatively large, and preferably, as large as possible. Currently, bias resistor values of about 10 Gohms are utilized. For further noise reduction, this value might be increased up to 100 Gohms. However, these resistors cannot be implemented on ICs but must be made on a thick film hybrid. As an alternative to the resistors for the preamplifier input impedance, a pair of small diodes coupled in parallel but in opposite polarity (i.e., “antiparallel”) are sometimes used. This permits a relatively large impedance value to be obtained, which value is strongly dependant upon process variations. However, the use of diodes for this purpose has the disadvantages of slow settling and slow overload recovery, which are characteristics of diodes.
In devices presently on the market, the bias voltage for the input of a preamplifier is supplied through a high-ohmic resistance or through a pair of anti-parallel diodes. In order to preserve the signal-to-noise ratio, the impedance of such device must be very high. On the other hand, the impedance has a certain maximum level, in order to enable a fast startup of the amplifier, and a fast recovery after overload. An integrated diode pair forms too high an impedance for guaranteeing stability; however, adding a parallel resistor (10–100 Gohms) introduces a level of noise.